It is known that banking establishments and the like use tamper evident containers for transmitting valuables, for example specified sums of money, securities, cash in transit (CIT), etc. from one department to another. The system operated by such establishments is such that it is readily possible to ascertain whether a tamper evident container has been stolen in transit. However it is ordinarily more difficult to ascertain whether the tamper evident container has been opened in transit and then resealed after some of the contents have been removed. In order to overcome this problem, tamper evident containers have been provided wherein any attempt to gain access to the interior of the tamper evident container becomes visibly apparent.
Typically, the walls of such tamper evident containers are formed of sheet material including a portion having an opening which gives access to the interior of the tamper evident container. The tamper evident containers include a closure portion arranged to be superposed on the portion having the opening, to close the tamper evident container. Closure is effected by means of a band of high-tack adhesive which is applied across the closure portion or the portion having the opening, for example from the molten state, from transfer tape, solvent cast or in the form of a tape. The adhesive may be pressure sensitive adhesive, and suitable adhesives include thermoplastic hot melt adhesives, silicone adhesives, acrylic pressure sensitive adhesives, solvent cast adhesives, UV (ultraviolet) or EB (electron beam) cured acrylic adhesives, and the like.
Such adhesives are required to have high initial tack with respect to the surface of the sheet material and also to have high adhesive and cohesive strength. In order to provide a visible indication of any attempt to open the tamper evident container by separating the closure portion and the portion having the opening, the adhesive should be strong enough to cause stretching, tearing, or other mechanical distortion of the portions upon attempted opening of the container. If desired, perforations or serrated edges may be provided in the closure portion to indicate tearing and emphasize the effect.
With the exception of silicone adhesives, adhesives suitable for the closure of tamper evident containers have a softening temperature which is below the melting point of the closure portion and of the sheet material. The softening temperature is commonly in the range of between 50.degree. C. and 90.degree. C. Accordingly, by the local application of heat, an unauthorized person can open and reseal the tamper evident container without any visible indication that the tamper evident container has been opened.
To discourage this practice, thermochromic inks have been used in tamper evident containers. These inks are formulated to develop a permanent, non-reversible, and visibly evident color change when the adhesive on the envelope is exposed to heating. In this way, if unauthorized access to e.g. a tamper evident container is attempted by means of local application of heat to an adhesive on the envelope, a color change in the ink makes this evident. Typical conventional thermochromic inks for this application are formulated from a leuco dye, a phenolic compound, an organic diluent, water, and polyvinyl alcohol. A thermochromic ink composition with improved wet abrasion resistance includes, in addition to the basic formulation of conventional inks, hydrolyzed poly(vinyl acetate) and an organic compound with at least one carbonyl group, preferably an aldehyde and more preferably a dialdehyde such as glyoxal. This improved ink composition is the subject of U.S. patent application Ser. No. 60/143211 assigned to a common assignee with the present application, and filed in the U.S. Patent and Trademark Office on Jul. 9, 1999, and refiled as a regular application concurrently with the present application.
Although these thermochromic ink systems provide a good visual indication of unauthorized tampering with the tamper evident container or other article, they typically require the end user to know in advance about the change in color; otherwise, the color change triggered by subsequent tampering activity may go unnoticed.
Also, such ink systems can be circumvented by unauthorized personnel by removing the thermochromic ink coating with an organic or aqueous solvent. To prevent this, further protection must be provided by printing messages over the thermochromic ink layer or coating, to prevent or make obvious any tampering of the thermochromic ink layer itself prior to heating.
It is therefore desirable to provide an article, especially an article such as a tamper evident container which utilizes thermochromic ink, which offers improved evidence of tampering.